Anyone on the streets past 9:00 p.m. could be arrested.
This passage clearly explains Martial law.
Step-by-step explanation:
It is evident in the passage that it describes a time of turbulence when the Major General Andrew Jackson comes to the town of the narrator and wants to have a look at the troops who are to be ready to fight the next day.
Thus, the line given here is pretty evidently showing what happens in Martial law that no one who gets out after that time in the evening is seen to be up to good and immediately held in arrest by the forces.